Power Coefficient Variation of an H-Darrieus Turbine with S1046 Airfoils at Different Tip Speed Ratios

Vertical Axis Wind Turbine (VAWT) efficiency is influenced by the airfoils´ geometry and its rotational speed. Previous research has been conducted to increase the Power Coefficient (Cp) using mainly NACA airfoils and reporting cycle-averaged performance parameters. However, the effect of the Tip Speed Ratio (TSR) on the instantaneous evolution of Cp and the Torque Coefficient (Ct) per cycle, as well as its relationship with the flow field in contact with the blades, has not been fully characterized. This paper presents a numerical study of the effect of TSR on the Cp and Ct of an H-Darrieus turbine using S1046 airfoils to identify the conditions associated with maximum Cp and explain the aerodynamic mechanisms governing turbine performance throughout a cycle. This evaluation was conducted using two-dimensional transient Computational Fluid Dynamics (CFD) in ANSYS Fluent. The numerical model's reliability was evaluated by comparing the predicted Cp-TSR values with those reported by Hashem and Mohamed, yielding relative differences ranging from 1.22% to 5.96% across the TSR ranges evaluated. The aerodynamic analysis included: turbulent Kinetic Energy (𝑘) contours, Pressure Coefficient (Pc), and velocity trajectory lines at key azimuthal positions of the rotation. It was found that Cp increases with TSR, reaching a maximum of 0.342 at a TSR of 2.25; as TSR continues to increase, Cp decreases. Flow field analysis showed that at a TSR of 2.25, there was better pressure distribution around the blades and a more uniform flow passage through the turbine, whereas at higher TSR values, the interaction between the blades and the flow was less efficient. The results show the effect of TSR on the performance of the rotating turbine throughout the aerodynamic analysis and serve as a reference for selecting the operating regime of H-Darrieus turbines.